Heretofore, as shown in FIG. 11(a), in a case for reducing a diameter an end portion of an elliptical hollow cylinder (a workpiece) 301 by spinning to form it into a circular hollow cylindrical shape, rollers 303 are contacted with the elliptical hollow cylinder 301 from outside.
However, according to this forming method, the rollers 303 cannot follow a shape of the elliptical hollow cylinder 301, and thereby the rollers 303 spin freely at short-diameter portions (see FIG. 11(b)). Namely, since a distance between a rotational axis of the elliptical hollow cylinder 301 and a rotational axis of the roller(s) 303 cannot be changed quickly, the roller(s) 303 contacts only with long-diameter portions and spins freely without contacting with the short-diameter portions.
If a rotational speed of the elliptical hollow cylinder 301 is made slow in order to make the rollers 303 capable of following the shape of the elliptical hollow cylinder 301, it takes more time for diameter reduction of the elliptical hollow cylinder 301 by spinning and thereby mass production becomes difficult. Therefore, a forming method for shortening time required for the diameter reduction of the end portion of the elliptical hollow cylinder 301 is proposed (see a Patent Document 1 listed below).
In the forming method disclosed in the Patent Document 1, first, a die (mandrel) 305 is inserted into an end portion 309 of an elliptical hollow cylinder 301 shown in FIG. 12, and the end portion 309 of the elliptical hollow cylinder 301 is formed into a circular hollow cylindrical shape as shown in FIG. 13. Then, as shown in FIG. 14, a diameter of the end portion 309 formed into a circular hollow cylindrical shape is reduced by spinning with a roller 307 to form a small-diameter end portion 309a. 